I am rebuilding a NiMH battery pack for a RF remote control device which can be recharged by plugging the device in.

The battery pack is connected with three wires:

  • black to the negative
  • red to the positive
  • BLUE through a mysterious discrete component to the negative

battery pack

The component in line with the blue wire looks a bit like a glass-body diode but my multimeter cannot measure a diode voltage. (It measures infinity, not zero, if this helps.)

However, I can measure about 11.5 kOhm across the component in both directions.

Can anybody enlighten me, what this component is likely to be? I would like to verify that the component is still functional because it is unclear whether or not the device still charges properly.

up vote 35 down vote accepted

Its a thermistor like this (photo from internet, not spam related):

Photo from internet

This kind of resistor depends on the temperature of both batteries.

EDIT: In this photo, is a Negative Temperature Coefficient. There are also Positive Temperature Coefficient resistors. The main difference between them is how the resistor decreases or increases when the temperature grows, respectively.

That's a cheap component and a cheaper way to determine when both batteries are charged up.

When a NiMH is almost charged, its temperature starts to grow. If you measure the temperature based on a voltage divider into a DAC, you can measure the temperature and, therefore, activate the charge or deactivate it.

EDIT: The thermistor must be very close to the batteries to read a correct value, so that's why this discrete component is located close to the batteries.

The temperature based graph to read when stop charging the battery is like this:

Photo from internet

Related: Voltage input for charging NiMH Batteries

If you can measure the temperature, you can check when it's charged up.

  • 3
    +1 your answer is much better than mine. – dim Nov 19 at 10:12

It is a thermistor. This senses the battery temperature, so the charger can know when something goes wrong.

Measuring it likely won't tell you if the battery pack is still working correctly, though.

  • 1
    Of course, that makes perfect sense. I should have thought of that. Easy to verify and not very likely to break. - Many thanks! – ARF Nov 19 at 10:11

Your Answer

By clicking "Post Your Answer", you acknowledge that you have read our updated terms of service, privacy policy and cookie policy, and that your continued use of the website is subject to these policies.

Not the answer you're looking for? Browse other questions tagged or ask your own question.